Well, i can tell you what you should NOT do, and that is to run the app from the .dmg file. I’ll tell you why later, but first, let’s get to the topic at hand.
I’m assuming that you are fresh from the Windows world, where downloading applications or programs simply meant downloading a .zip file with an .exe inside the archive.
.dmg files also work in a similar principle: after downloading the .dmg file, double click on it. OS X will verify the disk image and then mount it onto the system as what i would call a ‘virtual drive’. Inside this virtual drive contains one or more of the following:
1) The application, all self-contained inside its package.
2) An installer for your application.
3) An uninstaller.
It is quite easy to differentiate a self-contained application from an installer: the former is represented by its application icon, while the latter is usually represented by an installer icon, which looks like an open box with a huge cube inside of it.
To install a self-contained application into OS X, simple drag and drop the application into your ‘Applications’ folder (or anywhere else of your liking), and you’re done. Easy as pie.
To install an application that has its own installer, double click on the installer icon in the .dmg and follow the instructions. This one is similar to the Windows installation style you are familiar with, except that by default, the installer will dump your application into the ‘Application’ folder by default.
Either way, after you are done, you can unmount or eject the .dmg and trash it away: you won’t need it anymore. Of course, unless the .dmg file came with a bundled uninstaller. In that case, you should hold on to the .dmg file. Should you ever need to uninstall the application, you can make use of the bundled uninstaller to do so. I won’t recommend dragging out the uninstaller package from the .dmg file for a very simple reason: imagine having 5 or more uninstaller packages lying around your ‘Applications’ folder; hunting for the correct uninstaller will take considerable effort.
Now, you’re probably wondering: why shouldn’t i run my applications from within the .dmg file itself? Simply put: anything you work from within a .dmg will not be saved. So let’s say, if you just had a very nice conversation with your buddy on Adium and you want to save the conversation, but you are running Adium off the .dmg instead. Well, say goodbye to your precious log, because the next time you open Adium up from the .dmg, all your settings will have reverted to their defaults.
You’d probably be thinking: “But this is standard OS X operations! How can anyone NOT know?” Unfortunately, just because you and i know about it does not mean the next person does. And after seeing so many of my friends running their Firefox from their .dmg and complaining why all their important bookmarks are lost after the .dmg is ejected, i think a little basic Mac ‘how-to’ is in order.